This invention is related to solid fuel propellants, more particularly to hydroxy terminated polybutadiene based urethane binders therefore, processes for their cure and catalysts suitable for use therein. This invention is also related to polyurethane casting resins in general, catalysts for their cure, processes for their use, and cured articles formed thereby.
Composite solid propellants normally comprise a rubbery phase which is intimately admixed with, serving as a binder for, such additional solid ingredients as ammonium perchlorate or other oxidizers, finely divided aluminum, or other metallic fuel, and to a minor extent such other additives as, for example, iron oxide. The polyurethane binder system is known to impart superior physical and mechanical properties to the propellant. U.S. Pat. No. 3,245,849, the disclosure of which is incorporated herein by reference, is illustrative.
Very frequently the polyurethane binder will be one based upon hydroxy terminated polybutadiene. The prepolymer precursor for the polyurethane will then be a polymer having repeating butadiene units with terminal hydroxyls and a molecular weight of about 3000. In addition, hydroxyl groups are substituted along the chain so that the average number of hydroxyl groups per polymeric unit is in the range of from 2.1 to 2.7. The additional hydroxyl functions, of course, provide sites for cross-linking which is particularly important because the common polyisocyanates used for curing are difunctional.
As stated, curing is due to the reaction between the hydroxyl functions and the polyisocyanate. It is desirable that the speed of this curing reaction be controlled so that the mixture remains fluid [viscosity below about 40 Kp (pot life)] for a sufficient time to allow casting or other manipulation into the final desired form, and so that curing can then take place in a reasonable length of time under sufficiently mild conditions that no stresses or other mechanical defects are induced into the finished propellant grain, or that no excessive thermal input initiates undesirable chemical reactions among the highly energetic materials present. A shorter curing time of course also allows for savings in power consumption, greater put through capacities for cure ovens, and more rapid turn around for rocket hardware where reuse is possible. The use of catalysts to speed up the cure reaction is of course well-known, but at the temperature normally employed in processing and curing, normally between 63.degree. C. and 76.degree. C., to get adequate pot life, desirably in the area of about 8 to 14 hours. requires that the cure time then range from about 7 to 14 days.
The present invention enables the shortening of the cure time while allowing a desirably long pot life.
It has long been a general problem that in casting large polyurethane objects, that the difficulty in transferring large quantities of blended resin into the desired mold leads to inhomogeneities in the final product. Naturally once the cure agent is added to the mix, cure begins even while the material is being transferred from the mixing vessel to the mold. Material added early is thus less cured and more fluid than later added material. These changes in initial viscosity are translated into variation in properties in the final cured product. Attempts to avoid this problem by blending a system which is only slowly reactive at mixing and casting temperatures followed by thermal acceleration after completion of casting to complete the cure must contend with the expense of large scale heating equipment and the cost of the energy for its operation, the difficulty with getting uniform heat transfer throughout a large object, and the stress and strain induced upon cooling from the elevated temperature employed back to ambient levels. Both the lack of uniform heating and cooling stresses and strains will obviously lead to property variation throughout the final cured product.
The present invention enables the extension of the pot life for these systems also while allowing cure to take place without requiring application of excessive heat while permitting uniform cures in short time periods.